Soil Structure and Fabric

R Brewer, JR Sleeman

• Publisher: CSIRO PUBLISHING
• Year: 1988
• Print ISBN: 9780643048591
• eBook ISBN: 9780643106185
• Topic: Technology & Engineering
• Subtopic: Agronomy
• Index: Technology & Engineering

Soil Structure and Fabric sets out a method of describing the structure and fabric of soils that proceeds consistently from macroscopic descriptions in the field to descriptions from standard thin sections as viewed with a light microscope. Most of the book is devoted to the description and classification of the microscopic characteristics of soils.

Section 1

Classification of Soil Structures and Fabrics

Chapter 1

Framework for Description

1.1 The constituents of soils

Soils may consist of any of the constituents of rocks and their weathering products, constituents formed by reactions within the soil profile, and material from plants and animals. Compilation of a complete list of all the constituents that may occur is scarcely possible; the list would be impossibly long and, even now, probably not all of the constituents that do occur have been identified. It is appropriate, however, to consider the nature of the more common kinds, and their general properties, since interpretation of soil processes, and of the genesis of the structures and fabrics described later, depend on a knowledge of their properties and reactions (see Brewer 1972). The less common constituents are likely to have properties generally similar to one or another of the more common ones, at least in terms of their reactions to soil-forming processes, since the latter include inorganic and organic, crystalline and amorphous, and soluble^* and insoluble* constituents.

The major constituents can be grouped broadly as follows.

(1) Crystalline constituents: primary rock-forming minerals, layer silicates, chlorites, attapulgites, crystalline oxides (e.g. hematite, goethite, gibbsite), carbonates (e.g. calcite, dolomite), sulfates (e.g. gypsum, barite), and so on.

(2) Amorphous, inorganic constituents: amorphous oxides, the allophanes, and so on.

(3) Organic bodies: most commonly plant fragments, but also discrete plant cells, pollen grains, spores, sclerotia, pupal cases, and so on.

(4) Amorphous organic matter: amorphous, structureless organic material that occurs commonly within decomposing plant fragments and disseminated throughout the soil mass.

These groupings give some idea of the nature of the constituents that are involved in the fabrics and structures in soils. Living fauna are present (sometimes abundant) in some part of most soil profiles, but they are regarded as inhabitants of the soil rather than as constituents; they are rarely observed in micropedological studies because of the method of preparation of thin sections, which usually involves drying the sample and slow impregnation with a liquid resin. Similarly, living plant roots are not strictly a part of the soil even though they are commonly preserved in thin sections. However, the activity of fauna and plant roots, particularly the former, may have a profound effect on the morphology of soil materials.

Kubiena (1938) and Brewer and Sleeman (1960) recognized that the constituents of soils can be placed into two broad groups on the basis of their physical and physico-chemical properties, especially their reactivity due to particle size (clay-size versus larger grains) and solubility, which are relevant to their reactions to the processes of soil formation, thus:

(1) relatively unreactive constituents: constituents that are larger than clay-size and relatively insoluble;

(2) relatively reactive constituents: constituents that are of clay-size, amorphous or relatively soluble.

These groupings were the basis of definitions of skeleton grains and plasma, respectively, by Brewer and Sleeman (1960). Criticisms of these two concepts (e.g. Stoops and Jongerius 1975, 1977) indicate difficulties with the original definitions. Nevertheless, it is advantageous to be able to refer to such groups of constituents in discussion and in definitions of fabrics and structures. The following simplified definitions are designed for this purpose.

Skeleton grains. Simple crystals of minerals and individual organic fragments larger than clay-size; referred to as ‘mineral skeleton grains’ and ‘organic skeleton grains’, respectively [17].

Plasma. All the material, inorganic and organic, crystalline and amorphous, of clay-size; broad subdivisions are ‘inorganic plasma’, which includes crystalline (mineral) and inorganic amorphous plasma, and ‘organic plasma’ [17].

Some further comment about these groupings is desirable.

As in general pedology, it is convenient to set the upper limit of clay-size (the distinction between skeleton grains and plasma) at 2 μm (the International Standard); in the interests of brevity, clay-size, as used in this work, refers to crystalline constitutents smaller than 2 μm and/or amorphous inorganic and organic constituents.

There has been considerable discussion as to whether grains of the more soluble constituents (e.g. carbonates, sulfates) that are larger than 2 μm should be regarded as plasma because of their potential solubility (e.g. Stoops and Jongerius 1975, 1977; Brewer 1976). In practice, the argument is unimportant. They are classed here as skeleton grains, which is a divergence from the original concepts of Brewer and Sleeman (1960).

Skeleton grains, although relatively large, can be translocated and concentrated by some processes, as is evidenced by silt cappings on large mineral grains and rock fragments in some soils that freeze and thaw regularly, and by cracks and tubular voids that have been filled with sand-size grains by rather extreme down-washing. Also, plate-shaped skeleton grains (e.g. mica grains) can be re-organized by strong differential pressures and certain kinds of soil flowage to produce an observable pattern of preferred orientation similar to that which is more commonly observed due to orientation of clay-size layer silicates (mineral plasma), which are also plate-shaped.

Skeleton grains can weather to produce plasma. Weathering of, say, a mica grain may gradually change its optical properties throughout the whole grain and the end-product is a pseudomorph of, say, clay-size layer silicates (mineral plasma). For practical purposes, in view of the optical techniques used in micropedology, such weathered grains are regarded as skeleton grains up to the stage where they have lost the typical ‘mottled’ extinction of mica and their pleochroism, if originally present. In other instances, weathering of mica grains produces layers of clay-size minerals sandwiched between layers of relatively fresh mica. Such grains are regarded as skeleton grains as long as fresh layers of mica can be recognized optically. Other mineral skeleton grains also weather to produce plasma; using similar criteria as for the micas, weathered grains are regarded as skeleton grains as long as they are recognizable as grains and some parts of the grains have the optical properties of the original mineral. When weathering has proceeded beyond these limits, the pseudomorphs that may result are regarded as some variety of associated structure (Chapter 7).

Similar criteria can be applied to decomposed organic skeleton grains, and to weathered rock fragments. The former are regarded as skeleton grains as long as some part of their tissue structure can be recognized, and the latter as rock fragments (lithules, see Table 14) as long as the rock fabric can be recognized.

This grouping of constituents into skeleton grains and plasma is used mainly in discussions of the kinds of constituents involved in certain arrangements and features observed in soils. However, it is not necessarily the best grouping for all aspects of micropedology, in particular, for descriptions of fabric. Groupings made according to characteristics other than those used to define varieties of skeleton grains and plasma are more appropriate for description of many aspects of the arrangement of the constituents. Some of these are discussed later (Chapter 3.1).

Some of the constituents of soils are readily identifiable in thin sections, which provide the most useful data for studies of structure and fabric, but others are not, especially the constituents of the plasma (Appendix IV.1). However, the appropriate specificity of the identification of individual constituents depends on the purpose of the study. In general, for studies of structure and fabric, it is more important to identify those constituents that occur as concentrations in particular sites in the soil material, on the assumption that such concentrations may be the result of soil-forming processes and/or may be significant in the physical or chemical behaviour of the soil; concentration of a constituent, of course, facilitates its identification in thin section. It is generally less important to identify specifically the more stable, randomly distributed constituents, such as primary rock-forming minerals and rock fragments; it is often sufficient to recognize them as such.

These generalizations, of course, require qualification: the more detailed the study, the more specific the identifications must be. For some studies, it may be sufficient to record clay coatings on the walls of voids; in other cases, it may be important to know the kinds of clay-size minerals and the accompanying contaminants (such as clay-size iron oxides) in the clay coatings. Studies of weathering and provenance may require accurate identifications, and even quantitative estimations, of the rock-forming mineral species, rock fragments and their weathering products. Studies of the decomposition of organic materials may require identification of organic bodies and their composition in relation to their internal structure.

It is not proposed to examine here the details of the numerous methods of identification of soil constituents, but mention should be made of the techniques most commonly used and their suitability in studies of fabric and structure.

The most useful data are those obtained directly from thin sections because they show the distribution and relationships of the constituents. These are obtained optically with a petrological microscope, supported, if necessary, by scanning electron microscopy and analysis made on polished thin sections with an electron microprobe analyser (which is capable of elemental analysis of volumes as small as about 1 μm in diameter). The latter techniques require that the material being analysed be exposed at the surface of the section.

Somewhat less convenient is the X-ray microcamera, which can be used to obtain diffraction patterns from areas as small as 50 μm in diameter in peels taken from thin sections. Such peels can be examined optically before X-ray analysis, and re-examined when the results have been obtained.

Other techniques currently being applied to soil sections have been reviewed by Bisdom (1981). Other data are obtained from disturbed material by standard chemical and X-ray techniques. Interpretation of these data in terms of the distribution of the constituents in the soil, in relation to its structure and fabric, is made from thin sections by inference based on experience.

1.2 Structure, fabric and texture

Structure, fabric and texture involve various aspects of the size, shape and arrangement of the constituents, compound particles, and voids, and the crystallinity of the constituents. In this context, ‘arrangement’ is concerned with spatial relationships in terms of distribution patterns of the various constituents (and voids), packing, and orientation patterns of optically and/or physically anisotropic particles. ‘Crystallinity’ has to do with whether the constituents are coarsely or finely crystalline, amorphous, anhedral or euhedral, and so on. Various combinations of these characteristics have been used to define structure, fabric and texture as they are presently used in petrology and pedology. The several definitions of each of these concepts embody different shades of meaning, even though they are all based on combinations of the same characteristics (Appendix I).

The geological concept of structure is broader than the traditional concept of soil structure (Soil Survey Staff 1951), which is only one aspect of it. It deals with additional features such as bedding, banding, concretions, pisoliths, the nature of contacts between different kinds of materials, and so on; all of these features also occur in soils. On the other hand, the concept of fabric as used for igneous rocks, is very like that generally used in micropedology. Unfortunately, texture is used in pedology in a completely different sense to its use in geology (Appendix I) even though the geological concept is clearly applicable to soils. In the interests of uniformity, therefore, the major concepts of structure, fabric and texture used in this book are based on those used in petrology, as set out in the ‘Glossary of Geology’ (Gary et al. 1974). The concepts are defined as follows:

Structure. Structure is a megascopic feature, generally seen and studied best in an exposed section in the field, rather than in hand specimen or thin section, and representing a discontinuity or major inhomoge...

Table of contents

• Cover
• Title
• Copyright
• Contents
• Preface
• Introduction
• Section 1: Classification of Soil Structures and Fabrics
• Section 2: Systematic Nomenclature
• Section 3: Application to Soil Profiles
• Section 4: Tables and Schedules for Description and Classification of Structures, Fabrics, Somas and Soil Profiles
• Section 5: Plates depicting Structures, Fabrics and Somas
• Appendices
• References
• Index